Answer ILLEGAL_FUNCTION for a request with a reserved function code

pymodbus/pdu/decoders.py

@@ -68,6 +68,11 @@ def decode(self, frame: bytes) -> ModbusPDU:
         function_code = 0
         try:
             if (function_code := int(frame[0])) > 0x80:
+                if self.is_server:
+                    raise ModbusIOException(
+                        f"Reserved function code {function_code}",
+                        function_code=function_code,
+                    )
                 pdu_exp = ExceptionResponse(function_code & 0x7F)
                 pdu_exp.from_frame(False, frame)
                 return pdu_exp

test/framer/test_extras.py

@@ -142,7 +142,7 @@ def test_ascii_process_incoming_packets(self):
     def test_rtu_decode_exception(self):
         """Test that the RTU framer can decode errors."""
         msg = b"\x00\x90\x02\x9c\x01"
-        _, pdu = self._rtu.handleFrame(msg, 0, 0)
+        _, pdu = FramerRTU(DecodePDU(False)).handleFrame(msg, 0, 0)
         assert pdu
 
     def test_tcp_framer_skip_wrong_dev_id_then_decode(self):

test/pdu/test_decoders.py

@@ -132,6 +132,13 @@ def test_server_decode(self, code, frame):
         assert pdu
         assert pdu.function_code == code
 
+    @pytest.mark.parametrize(("code", "frame"), list(exceptions))
+    def test_server_decode_reserved_function_code(self, code, frame):
+        """A request may not use a function code reserved for exception responses."""
+        with pytest.raises(ModbusIOException) as exc:
+            self.server.decode(frame)
+        assert exc.value.fcode == code
+
     @pytest.mark.parametrize(("frame"), [b"", b"NO FRAME"])
     @pytest.mark.parametrize(("decoder"), [server, client])
     def test_decode_bad_frame(self, decoder, frame):

test/pdu/test_pdu.py

@@ -30,7 +30,7 @@ async def test_error_methods(self):
         """Test all error methods."""
         result = self.exception.x_encode()
         assert result == b"\x01"
-        DecodePDU(True).decode(b"\x81" + result)
+        DecodePDU(False).decode(b"\x81" + result)
         assert self.exception.exception_code == 1
         self.exception.x_decode(b"")
 

test/server/test_server_asyncio.py

@@ -274,6 +274,14 @@ async def test_async_tcp_server_write_coil_illegal_value(self):
         await asyncio.wait_for(BasicClient.done, timeout=0.1)
         assert BasicClient.received_data == b"\x00\x00\x00\x00\x00\x03\x01\x85\x03"
 
+    async def test_async_tcp_server_reserved_function_code(self):
+        """A function code reserved for exception responses must answer 0x01."""
+        BasicClient.data = b"\x00\x00\x00\x00\x00\x04\x01\x81\x00\x00"
+        await self.start_server()
+        await self.connect_server()
+        await asyncio.wait_for(BasicClient.done, timeout=0.1)
+        assert BasicClient.received_data == b"\x00\x00\x00\x00\x00\x03\x01\x81\x01"
+
     async def test_async_server_trace_connect_disconnect(self):
         """Test connect/disconnect trace handler."""
         trace_connect = mock.Mock()